The present invention relates generally to cross-polarized satellite transmission systems, and more specifically to an uplink cross-polarization interference canceller.
Orthogonal cross-polarization technique is employed in satellite communications systems to substantially double the bandwidth of transmission channels of the systems to achieve full utilization of satellite channel resources. With the known cross-polarization technique, two radiowaves of the same frequency are orthogonally cross-polarized with respect to each other to carry different information. Therefore, ambiguities must not exist to allow discrimination between the two cross-polarized signals. However, various factors affect on such signals. Differential phase and/or attenuation of microwave energy is one of such factors which is caused when a signal propagates pass through elliptically-shaped raindrops, resulting in the generation of undesirable cross-polarization components, or interference.
Two methods are currently known to cancel the cross-polarization interference in the uplink of a satellite communications system. One is a pilot control system and the other is a correlation method. While these techniques are fully described in a Japanese-language publication "Research on International Communications", Apr., 1981, pages 87 et seq.), a summary of these techniques will be given below with reference to FIG. 1.
In FIG. 1, the earth station transmits a first pilot signal f.sub.0 (L) on a left-hand circularly polarized plane and a second pilot signal f.sub.1 (R) on a right-hand circularly polarized plane. If the transmitted signal passes through a rainfall region, differential phase shift and/or attenuation occurs, so that portion of the signal on each polarization plane is rotated, resulting in the generation of interference components (f.sub.0 (R), (f.sub.1 (L)). The satellite transponder provides frequency translation on received signals to a downlink frequency as well as polarization translation. If the received signal is a right-hand circularly polarized wave, the satellite transponder would transmit it to the downlink of another earth station. Namely, the right-hand circularly polarized components (f.sub.1 (R), (f.sub.0 (R)) is not returned to the own earth station and the left-hand circularly polarized components (f.sub.0 (L), (f.sub.1 (L)) are returned as signals (F.sub.0 (R), F.sub.1 (R)) to the own earth station after frequency and polarization translation processes. Since the pilot signal at frequency f.sub.1 is a carrier-suppressed component of the other pilot signal at frequency f.sub.0 according to the known technique, the same modulating signal as that of transmission is used to detect the phase and amplitude differences between the principle component F.sub.0 (R) of the pilot signal and the cross-polarization component F.sub.1 (R) by a synchronous detector. Based upon these differences, rotatable 90.degree. and 180.degree. phase shifters of the transmission circuit are controlled.
However, since the uplink's right-hand circularly polarized signal does not return to the own earth station, the current pilot method is not applicable to all satellite communications systems. More specifically, if the F.sub.1 (L), which is a polarization translated version of the f.sub.1 (R) component, is to be returned to the own earth station, such a returning signal will undergo polarization of opposite direction if it passes through a rainfall area and results in an undesired component F.sub.1 (R) which mixes with the F.sub.1 (R) desired component. In addition, the pilot control method requires two pilot signals to be transmitted to two satellite translation circuits which respectively perform frequency and polarization translation and the earth station must be provided with high power amplifiers dedicated respectively for the two pilot signals in order to maintain them in a proper phase relationship. Further, the synchronous detector must be of high precision type. In addition, two transponders of the same frequency but opposite polarizations must be secured. This adds to the cost of maintenance.
On the other hand, the correlation method involves detecting the amplitude and phase information of the downlink cross-polarization interference from a beacon signal transmitted constantly from the satellite and estimating the cross-polarization interference of the uplink from the correlation between the downlink and uplink of the system to control the two phase shifters of the transmission circuit. Since interference cancellation is effected in an open loop on the basis of the correlation between signals on the downlink and those on the uplink, it lacks precision as compared with the cancellation method using two pilot signals.